Time to have a little fun. At the southern end of the gate, you mentioned there were new cultists being trained. They're almost certain to be holding a special item given to all initiates - an Orb of Illusion.

These artifacts are used to disguise a cult member, allowing them to gain safe passage, infiltrate, or simply blend in with an enemy.

Kill the fools until you find one, <class>, then use it on a subjugator.